Many electrical conductors are insulated with a layer of polyethylene material. Typically, this is done on a large scale by extruding the polyethylene onto the conductor as it passes through an extruder head. The speed at which the extrusion takes place may be in the order of a hundred feet per minute. This does not allow much time for in situ inspection of the resulting extruded insulation layer. Although the polyethylene insulation offers an excellent dielectric strength material, its effectiveness may be reduced by the presence of foreign objects such as dirt, carbon or other material with different dielectric strengths. The presence of these imperfections, which may be quite small, in the magnitude of 0.005 inch diameter or larger, causes localized electrical stresses within the insulation when voltage is applied to the cable. These resulting stresses create the well documented "treeing effect" in the insultion causing premature breakdown of the insulation and shorting of the cable life.
This problem of contamination has been of great concern to the electrical cable industry and a number of solutions have been proposed. One solution has been to have the material supplier inspect the polyethylene pellets prior to shipping to the cable manufacturer. However, their inspection techniques are subject to statistical error such that, for example, contaminants in the 0.004 inch range up to 0.010 inch range may be detected at a rate of only about 20 percent. This leaves a much larger amount of the contaminant behind in the raw polyethylene. In addition, such inspection techniques, even if 100 percent successful, are performed prior to shipping and extrusion onto the cable; while both the shipping and extrusion processes offer considerable opportunity for even further contamination.
A second technique which has been adopted by the Association of Edison Illuminating Companies (AEIC), is to inspect the insulation from the completed cable by physically excising a two-inch sample of completed cable for each ten thousand feet produced. These two-inch sections would then be sliced into wafers approximately 25 mils thick and physically examined under 15 power magnification for particulate contamination. This technique is also subject to much statistical error and may allow fairly large segments of contaminated cable to pass through undetected. In addition, this is a time consuming and labor intensive technique, which may be further affected by the proposed rules to increase the quantity of insulation inspected to 2 percent of the total insulating material prior to application to the cable.
The two-inch sampling technique mentioned above has a further drawback in that since this inspection takes place after the cable has been extruded, a great deal of money, time and resources has been expended on the production of this cable only to result in scraping the cable should a contaminated portion be found.
A further developement in contamination detection is an attempt to mechanize the previous inspection technique. In this technique, the insulation is inspected by directing a laser beam through the insulation and detecting the light scatter caused by the particle inclusions, thereby determining the amount of contamination present. A number of these systems have been designed for Electrical and Power Research Institute (EPRI) and are subjects of U.S. Pat. Nos. 4,208,126; 4,265,545 and are incorporated herein by reference. Unfortunately, these techniques require sophisticated laser tracking equipment and at the present are not cost effective.
Therefore, what is needed in the art is a cost effective method for detecting contaminants in the polyethylene insulation which would be representative of the insulating material in the as-applied condition and would allow for easy location of that contaminated portion of the cable simplifying removal of that portion.